After much back and forth, I found that the DG on it's highest amplification + the high gain of the O2, gave the most punch and authority levels; however as mentioned before induced a premature distortion on SOME audio playback. It seems the distortion is a combination of poor recording quality and equalizing that I have done, where the majority of that comes from the recording quality. Switching to medium level amp output on the DG aided in removing the distortion for those acute situations while keeping the EQ.

I also find that games and some film/tv, seem to have more natural volume to them - potentially a +gain that's hardcoded, however this does not induce any noticeable distortion so I simply turn the volume down a bit.

My conclusion - as discussed in the K612 owner's thread - is that the K612 needs some serious amplification to make it loud; more so than the usual recommendation we get from people that they are 'hard to drive,' and 'need synergy.'

I do not know if double amping at high gain and max output will damage the hardware, though I do not have much of a choice.

Ultimately, do not make the mistake I made before and assume that a simple portable type amplifier, such as a Fiio E11 or similar, is what people mean when they suggest you purchase a headphone amp.